electrohydrodynamics

Weakly conducting dielectric solid particles and liquid droplets in strong electric fields are known to undergo symmetry-breaking bifurcations leading to steady electrorotation. This so-called Quincke effect, which results from the antiparallel electrostatic dipole induced by the applied field inside the particles, is well described by the classic Taylor-Melcher leaky dielectric model.

Transfer of liquid from one surface to another plays a vital role in printing processes. During liquid transfer, a liquid bridge is formed and subjected to substantial extension, but incomplete liquid transfer can produce defects that are detrimental to the operation of printed electronic devices. One strategy for minimizing these defects is to apply an electric field, a technique known as electrostatic assist (ESA). However, the physical mechanisms underlying ESA remain a mystery.

A classic result due to G.I.Taylor is that a weakly conducting drop bearing zero net charge placed in a uniform electric field adopts a prolate or oblate spheroidal shape, the flow and shape being axisymmetrically aligned with the applied field. Here I will overview some intriguing symmetry--breaking instabilities (Quincke rotation resulting in drop steady tilt or tumbling, and pattern formation on the surface of a particle-coated drop), and the streaming from the drop equator that creates visually striking “Saturn-rings” around the drop.

Electrohydrodynamics (EHD) of viscous drops and vesicles is of great relevance in biomedical, engineering and industrial applications. In this talk I will focus on results on this subject from my collaborations with various groups over the years. Modeling results are presented from investigating the EHD of vesicle and viscous drop under an electric field.

Recent work has demonstrated the interesting dynamics possible when considering multicomponent vesicles. Up to now, the dynamics of two dimensional vesicles have been studied. In this work, the dynamics of fully three-dimensional, multicomponent vesicles in the presence of electric fields will be investigated. Building upon a volume and surface area conserving Navier-Stokes projection method, the appropriate forcing terms are derived from the energy of a multicomponent membrane. The evolution of the surface components is done via a conserving, surface Cahn-Hilliard model.

The deformation of a weakly conducting, leaky dielectric, prolate drop in a density matched, immiscible weakly conducting medium under a uniform DC electric field is analyzed. Using boundary integral computations, we delineate drop deformation and breakup regimes in the Ca_E-Re_E parameter space, where Ca_E is the electric capillary number (ratio of the electric to capillary stresses); and Re_E is the electric Reynolds number (ratio of charge relaxation to flow time scales), which characterizes the strength of surface charge convection along the interface.

We present a boundary integral equation method, developed recently, for simulating the coupled electro- and hydro-dynamics of vesicle suspensions subjected to external flow and electric fields. The dynamics of a vesicle are characterized by a competition between the elastic, electric and viscous forces on its membrane. The classical Taylor-Melcher leaky-dielectric model is employed for the electric response of the vesicle and the Helfrich energy model combined with local inextensibility is employed for its elastic response.

Colloidal particles trapped at the interface of a drop assemble in various dynamic patterns, e.g., vortices, when an electric field is applied. In this talk I will overview our experiments exploring the various particle structures, and the theoretical analysis of particle motion along the fluid interface.